Electrolytic capacitors

ABSTRACT

The present invention relates to electrolytic capacitors containing an electrolyte which is substantially nonaqueous, in which said electrolyte contains a dissolved chemical compound capable, as a result of the high-operating temperatures of the capacitor, of decomposing while at the same time giving off water and leaving a residue which remains dissolved, both the proportion and the solvent of this compound being so chosen that the dehydration of the latter substantially compensates for the water loss in the capacitor without affecting the electrochemical functioning of the electrolyte at these temperatures.

United States Patent Chesnot [451 Jan. 25, 1972 1 ELECTROLYTICCAPACITORS I References Cited [72] Inventor: Bernard Francois GustaveChesnot, Paris, 7 UNITED STATES PATENTS 3,546,119 12/1970 Chesnot..252/62.2 [73] Assignee: Les Condensateurs Sic-Safco, Colombes,

France Primary Examiner-Tobias E. Levow 1 Assistant ExaminerJ. Cooper[22} 1970 Attorney-Waters, Roditi, Schwartz & Nissen [2l] Appl. No.:24,994

[57] ABSTRACT The present invention relates to electrolytic capacitorscon- Rellled Application Dam taining an electrolyte which issubstantially nonaqueous, in [63] continuationdmpan of ser No 701075 Jan29 which said electrolyte contains a dissolved chemical com- 1968 Pat NO3 546 l 19 Q pound capable, as a result of the high-operatingtemperatures Q of the capacitor, of decomposing while at the same timegiving 52] U S Cl 317/230 off water and leaving a residue which remainsdissolved, both [51] In} .Cl 189/02 the proportion and the Solvent ofthis compound being so [58] Field 252/622' 317/239 hosen that thedehydration of the latter substantially compensates for the water lossin the capacitor without affecting the electrochemical functioning ofthe electrolyte at these temperatures.

4 Claims, 2 Drawing Figures temperatures of the order of 125 C. whilenevertheless retaining completely stable properties, the said capacitorsbeing additionally capable of withstanding very low temperatures of theorder of -5 5 C.

In order that an electrolytic capacitor containing a substantiallynonaqueous electrolyte should be able to operate at 125 C., theelectrolyte obviously cannot contain a large quantity of water, althoughwater must nevertheless be present in a small quantity to reform theanode layer, as is demonstrated by the following experiment which led tothe discovery from which the present invention originates:

If a test specimen consisting of a smooth sheet of aluminum is anodizedin a bath composed of melted triisopropanolamine borate maintained at200 C., it will be observed that alumina anode films can be formed up tovoltages of around ,600 v. However, after a certain number of testspecimens havebeen produced it is found that such formation is no longerpossible and that the aluminum test specimen becomes covered .withbrownish streaks, while the electric current remains at a high andconstant value.

Physical and chemical examinations then show that none of the propertiesof the triisopropanolamine borate, such as its melting point or color,have altered, and that it has not decomposed. lt is then observed thatthe normal formation process can be resumed by adding to the molten batha small quantity of boric acid which dehydrates into metaboric acid atthat temperature, but that if metaboric acid is added directlyjnstead ofboric acid, the formation process continues to be impossible. Thisproves that it is indeed thesmall quantity of water liberated by thedehydration of the boric acid that permits anode formation.

During protracted operation of the capacitor at 125 (1., this vitallittle quantity of water tends to disappear by evaporation andelectrolysis. it is then extremely important for the electrolyteformulation to be such as to ensure that the initial water balance ofthe essential constituents of the capacitor is retained as far aspossible at such temperatures. Otherwise the result will be eitherpremature destruction of the capacitor through self-sustained elevationof the leakage current, or an unacceptable drift in the electricalcharacteristics of the capacitor.

ln accordance with the present invention, the water content of theelectrolyte is maintained approximately constant by incorporating in theelectrolyte a chemical compound which, at temperatures on the order of125 C., decomposes and in so doing gives off water and leaves a residuewhich remains dissolved without affecting the electrochemicalfunctioning of the electrolyte, both the proportion and the solvent ofthe said compound being so chosen that the dehydration of the, lattersubstantially compensates for the water losses in the capacitor. 1

A chemical compound particularly suitable for this purpose is boricacid, which dehydrates into metaboric acid in accordance with thereaction:

By way of example, 'y-butyrolactone, 'y-valerolactone andN-methylpyrrolidone can constitute boric acid solvents capable ofachieving the aqueous balance in accordance with this invention.

The annexed drawing shows schematically in FIG. 1, an example of a coilof electrodes and insulating sheets between said electrodes in anelectrolytic capacitor, said coil being partially open in FIG. 2 asection diametrically made in a finished capacitor.

In the represented embodiments, electrodes 1 and 2 are of conventionalconstruction, smooth or etched aluminum sheets; one of the twoelectrodes forming an-anode is an aluminum sheet, covered by anodicoxidation of a skin of oxide taking the role of adielectric in saidcapacitor; porous separators 3, 4, 5 and 6 maintain electrodes inseparated situation and are used as reservoir for electrolyte,connecting lugs 7 and 8 .being used for electrically connecting saidelectrodes to terminals, the anodic lug being compulsorily made of avalve metal (aluminum, for example).

On-FlG. 2is shown a conventional section of a capacitor includinga coilas shown in FIG. 1. An aluminum box 9, obtained by extrusion, andresistant is used. as a container for said with anelectrolyte-impregnated coil, said box being provided with a groove 10forming an abutment for an obturator ll fastened thereon, leaktightnessbeing obtained by a joint 12, generally in an annulus 12 of elastomericsubstance.

Connection lugs 7 and .8 are mechanically connected thereon by solderingor riveting in 13 and 14 to the terminals 15v and 16; coil is maintainedin its situation by stars 17 and 18, made of polypropylene, for example,constituted with three fingers. A most particularly preferredelectrolytic system for electrolytic capacitor is hereinbelow indicatedby the following examples. According to the invention there areobtainable suchelectrolytic capacitors of the aluminum type comprising asubstantially nonaqueous electrolyte which enables anodic formation upto 200 C., by the effect of the thermal dehydration of the boric acidinto metaboric acid as the sole source of water, at a rate such that thecompensation for, the loss of water is effective at all temperatures,without the occurrence of secondary reactions causing a drift of theelectrical characteristics of the capacitor, such electrolyte consistingessentially of boric acid dissolved in a solvent selected from the groupconsisting of 'y-butyrolactone, y-valerolactone and N-methylpyrrolidone, said boric acid being present in an amount of 0.5 to10 moles per 1 1 moles of solvent.

EXAMPLE 1 An electrolyte capable of being utilized up to a maximum of400 volts can be produced on the basis of the following formulation inwhich the components are given in terms of moles Butyrolactone l lEthylene glycol 0.80 Boric acid 2 Tributylamine 0.36

the proportion of boric acid lying between 0.5 and 10 molecules butpreferably between 1 and 4 moles and the proportion of tributylaminelying between 0.1 and 2 moles, but preferably between 0.2 and 1molecule.

Ethylene glycol or another makeup solvent is often necessary in smallquantities for reasons of solubility.

In order to lower its resistivity, the composition may have added to itother anion-generating agents chosen from among acetic, acrylic,butyric, citric, isovaleric, lactic, maleic, phosphorous, phosphoric,propionic, tartaric and n-valeric acids. The unexpected observation isthen made that the stability of the characteristics is further improvedat C., presumably because these other anion-generating agents do notengage in a reaction which is liable to modify them, whereas the boricacid very slowly transforms itself into metaboric acid at thistemperature. a

This being so, the previously mentioned limits for the formulation willbe modified as follows for l 1 moles of butyrolactone: from 0.1 to 10moles (but preferably from 0.2 m4 moles) of boric acid, from 0 to 4moles for the secondary anion-generating agent or agents and from 0.1 to4 mole for the cation-generating agent or agents.

The nature of the cation-generating agents is not of primary importanceprovided that it or they remain stable at 125 C. and produce, with theanion-generating agents utilized, compounds which are soluble in thesolvents used; tributylamine and triethylamine, for example, meet theserequirements.

The formulation given in example 1 above can undergo very manymodifications, examples 2 and. 3 hereinbelow being given merely toillustrate this.

EXAMPLE 2 Butyrolactone ll moles Ethylene glycol 0.80 Boric acid 1.9Acetic acid 0.1 Tributylamine 0.36

The addition of acetic acid improves the properties of the electrolyteinsofar as the danger of crystallization at -55 C. is concerned. Thelongevity at 125 C. is at least equal to that of the electrolyte inexample 1.

EXAMPLE 3 Butyrolactone ll moles Ethylene glycol 0.80 Boric acid 1.0Lactid acid 1.8 Phosphoric acid 0.02 Triethylamine 0.80

The incorporation of lactic acid and phosphoric acid distinctly improvesthe constancy of the impedance at the different temperatures, though themaximum service voltage is lowered to 50 volts.

it goes without saying that, without departing from the scope of theinvention, changes may be made in the exemplary formulations givenhereinabove. To clearly illustrate the practical advantages of thepresent invention, mention will be made of a test in which the windingsof electrolytic capacitors of 20 ii/100 volts were dipped inelectrolytes Nos. 1 and 2, placed in cylindrical containers 11 mm. indiameter and 32 mm. long, sealed with a synthetic elastomer and reformedfor hours at 85 C. they were then energized with their nominal 4 voltageat 125 C., whereupon a 10 percent negative drift in the capacitancevalue was noted, while the loss factor (measured at cycles per second at20 C.) rose from 0.06 to 0.09 in 5,000 hours in the case of electrolyteNo. 2, as against 2,000 hours in the case of electrolyte No. l. i

In a second test involving capacitors of smaller size (6.35 mm. indiameter and 18 mm. long) rated at 3 utI/lOO volts and impregnated withelectrolyte No. 2, the same drifts in the characteristics as in thefirst test were noted after 3,500 hours of testing at C.

What I claim is:

I. An electrolytic capacitor comprising a plurality of spaced, aluminumelectrodes, and a substantially nonaqueous electrolyte in contact withsaid electrodes consisting essentially of boric acid dissolved in asolvent selected from the group consisting of 'y-butyrolactone,'y-valerolactone and N- methylpyrrolidone in an amount of 05-10 molesper 1 1 moles of solvent.

2. An electrolytic capacitor as claimed in claim I, wherein theelectrolyte further contains 0.8 mole of ethylene glycol per l 1 molesof solvent as a solubilizing agent.

3. An electrolytic capacitor as claimed in claim 1, wherein theelectrolyte further contains an aniogenic substance selected from thegroup consisting of acetic, butyric, lactic, maleic, phosphoric,propionic and tartaric acids in an amount of up to 4 moles per 1 1 molesof solvent.

4. An electrolytic capacitor as claimed in claim 1, wherein theelectrolyte further contains a cationogenic substance selected from thegroup consisting of tributyland triethylamine in an amount of 0.1-2moles per 1 1 moles of solvent.

2. An electrolytic capacitor as claimed in claim 1, wherein theelectrolyte further contains 0.8 mole of ethylene glycol per 11 moles ofsolvent as a solubilizing agent.
 3. An electrolytic capacitor as claimedin claim 1, wherein the electrolyte further contains an aniogenicsubstance selected from the group consisting of acetic, butyric, lactic,maleic, phosphoric, propionic and tartaric acids in an amount of up to 4moles per 11 moles of solvent.
 4. An electrolytic capacitor as claimedin claim 1, wherein the electrolyte further contains a cationogenicsubstance selected from the group consisting of tributyl- andtriethylamine in an amount of 0.1-2 moles per 11 moles of solvent.